Persons, when in dark areas that are not provided with lighting installations, or if these are provided, then when there is a power failure, or in circumstances where the individual prefers not to turn on the lights, feel the need for a portable light-weight light source for localized illumination. With it, they can illuminate areas of concern for their own protection and guidance. The response to this requirement is the common flashlight or the directed-beam lantern.
The most common flashlight is designed to provide a focused or small area beam, commonly called a "spot" beam. This is intended to be a relatively high-intensity beam with a limited area of illumination. Its preferred pattern, at least at its center, is a circular disc of reasonably uniform intensity. Another common objective is to provide a broader beam, that is, a beam with a larger illuminated disc. For the same luminous output from the light source, its intensity will be less than that of the smaller-area spot beam by the ratio of the two areas.
For an ideal point light source, it is possible, of course, to design a reflector to produce a beam of any given desired diameter which is collimated and consequently does not increase with distance to the illuminated area. However, this requires a reflector configuration respective to each beam size. Furthermore, the parameters of such a reflector require an increasingly larger reflector as the diameter of the focused beam increases. These reflectors would be paraboloids of varying sizes. This requirement for larger size reflectors in order to produce larger size beams is a serious design limitation. In response, reflectors of various configurations have been suggested to produce broader beams with smaller reflectors by displacing the light source along the axis of the paraboloid. Still, the consequence of such designs has been a unique pattern at some established distance. At different distances, the pattern has undesirable variations and distortions since the reflected light rays are not parallel to the axis of the reflector. In general, such variations are often characterized by dark regions in the areas of the beam of greatest interest to the user.
One further limitation of the conventional flashlight prevents the formation of an "ideal" spot beam that has the same diameter as the maximum diameter of the reflector. Conventional flashlights employ a polished surface paraboloidal reflector with a hole at the apex to accommodate the light bulb and bulb support structure. The result of having no reflecting surface near the centerline of the reflector is an unilluminated center disc when the point light source is positioned at the focus of the paraboloid. In order to illuminate the center area, the light source must be moved off of the focus and consequently produces a spot beam that has a larger diameter than the axis or diameter of the reflector.
There remains to be provided a flashlight which can selectively produce, with a relatively small reflector, both a small spot beam and a larger area broad beam, with a reasonably constant luminosity across the beam in both beam configurations over a substantial range of distances. It is an object of this invention to provide such a flashlight.
It should be kept in mind that the common flashlight has an incandescent filament and a concave reflector. Light emitted by the filament exits the flashlight in two modes. One mode is that of radiantly-emitted light without reflection. The reflector has an aperture which serves as a cut-off for this direct radiant illumination, and this light is emitted generally as a cone, and provides general low-level illumination, even outside of a central area yet to be described. The intensity of this direct radiated light decreases very rapidly at distance from the bulb because it is not collimated or controlled as is the reflected light. While substantial, the "conical volume" of this illumination from the filament is considerably less than the reflectively projected light which is reflected by the reflector in a designed, directed, pattern. It is the reflected projected light which provides almost all of the useful illumination from the flashlight. This useful illumination is the combination of light from the filament which goes reversely to the reflector, and also light which goes forwardly and still meets the reflector.
It is an object of this invention to provide a reflector which can project the light in either a spot beam or in a broad beam, both of which beams will be without substantial unilluminated areas over a substantial range of distances. It is a matter of great frustration with a conventional flashlight to find that, in the broad beam setting, the area of greatest interest is also that of darker or little illumination.
There is yet another problem with the common flashlight. Conventional reflector design is based upon the concept of a point source of light, and a focal point of a reflecting surface of revolution, usually a paraboloid. This is good theoretical geometry, and flashlights designed this way are sold by the millions. The imperfections of their projected light patterns have been overlooked in the absence of a better alternative.
The major problem in designing a flashlight which can produce both a spot beam and a broad beam is that the spot beam is best provided by a parabolic reflector with the light source close to the focus of the paraboloid. In order to broaden the beam, the light source is moved further away from the focus. This movement, depending upon its magnitude and the distance between the flashlight and the illuminated area, results in distortions such as unilluminated regions, usually in regions of greatest interest.
Conventional reflector design generally ignores these variations, sometimes by changing some areas of the reflector from a smooth surface of revolution to ones which include small discrete flat surfaces, or to an "orange peel" texture. These alterations serve largely to disguise the shortcomings of the reflector by scattering or diffusing some of the light. This is done at the trade-off cost of reducing the intensity of the light where it is needed the most.
Another major problem which is generally overlooked is that the filament of the conventional light bulb is not a point source. Instead, it is a curved line source, and therefore cannot be a point source anywhere. Even worse, not only must it inherently extend laterally from the central axis of the reflector, but due to variations in manufacture, no part of it at all may actually be on the central axis. Because the dimensions of the usual reflector are relatively small, even very small excursions of the filament from the central axis result in substantial deformations of the projected light pattern. In fact, in typical flashlights, a shift of only 0.1 inches along the axis is required to change from spot beam to broad beam, and even smaller ones in radial directions result in substantial distortions of the projected pattern.
The conventional adjustable or fixed beam flashlight has a flashlight axis related to a support such as its handle. Usually there is a method to align both the bulb axis and the reflector axis to the flashlight axis. The classical solution is to provide registry surfaces to hold both the bulb and the reflector in line as a single adjustment so they cannot be moved radially or angularly from the registered position. If the conformation of the bulb and of the reflector relative to the registry surfaces are both exact, all is well. However, this is rarely the situation. This is because the light bulbs used in these flashlights have a filament, usually a coil with a finite length, some or all of which is certain to be disposed off of the axis of the bulb, and if it is, then it certainly will be off of the axis of the reflector. The result is that although the bulb axis and the reflector axis are aligned, the filament is misaligned. The registry surfaces prevent any adjustment either radially or angularly. As a result of the filaments being off of the reflector axis and of having a finite size, the conventional paraboloidal reflector produces a spot beam that is irregularly shaped and has dimensions considerably larger than the maximum diameter of the reflector. The result is a duller spot beam than an ideal spot beam of smaller size. In addition, the adjustable beam flashlight with a paraboloidal reflector will produce a broad beam with an illuminated ring of light surrounding an unilluminated center disc precisely where the flashlight is aiming and in the region of greatest interest. These faults and the methods to overcome them will be made clear by the teachings of this patent.
It is an object of this invention to provide means to improve the distribution of reflectively projected light by adjustably positioning the filament in a uniquely contoured reflector.